1. Field of the Invention
The present invention relates to a storage device which reproduces/records information from/to a storage medium, a tracking control method, and a tracking control unit thereof.
2. Description of the Related Art
In a storage device such as an optical disk unit and a magnetic disk unit, a tracking control method is used where a relative position between a read head and a target track is detected and the signal is input to a head drive unit via an analog or digital controller, so that the head follows up the positional changes of the track.
As the price of storage device comes down, decreasing the number of components and the number of manufacturing steps is demanded for the disk unit. For this, a precision/coarse integrated type drive unit, where a precision actuator and a coarse actuator are not separated, is often used to configure a head drive unit.
For example, in order to implement low cost in the tracking control system of an optical disk unit, it is effective to perform tracking control and access control of the optical head by the thrust of a common coil. In other words, a precision actuator (used for tracking control and having a narrow movable range) and a coarse actuator (used for access control and having a wide movable range) are not disposed separately, but one actuator is used for driving for both precision and coarse controls, so that equipment cost can be decreased. A configuration example of equipment where tracking control and access control can be performed by one actuator, as mentioned above, has been disclosed in Japanese Patent Laid-Open No. S63-224037.
However, if a precision/coarse integrated type drive unit is used, robustness against the higher resonance of the head and follow-up to the eccentric vibration of the medium in a low frequency area must be implemented by only one feedback control unit.
It is difficult to implement these two requirements with the configuration of a general feedback controller. Because if a gain at a high frequency area is decreased to maintain robust stability against higher resonance, a phase near gain cross-over frequency delays, and the control band cannot be sufficiently increased.
For a feedback control unit, a digital filter using such a processor as DSP (Digital Signal Processor) is frequently used. In this case, the digital filter can calculate only at each sampling time Ts, so delay Ts/2 is generated to the tracking control system. The phase delay due to this delay time is also a cause which makes an increase in the control band difficult.
In other words, in the case of the above mentioned actuator which can control driving for both precision and coarse control, generally it is difficult to increase higher resonance frequency, so a gain of tracking control cannot be increased (gain cross-over frequency cannot be increased), and it is hard to support high-speed disk rotation.
To prevent the influence of higher resonance, it is possible to use a configuration where a twin T filter (notch filter) is inserted near the higher resonance frequency of the actuator. However, if a twin T filter where the dip frequency is low (close to the gain cross-over frequency) is inserted into the loop to be controlled, a large phase delay is generated at the gain cross-over frequency by the twin T filter, and phase margin decreases.
A method to solve this problem is stated in Japanese Patent Laid-Open No. H5-47125. In other words, an appropriate signal is input into the servo loop from outside, a resonance frequency is determined by the response of the servo system to the signal, and the notch filter is configured such that gain at this frequency becomes the minimum. With this method, the characteristics of the notch filter are optimized even if a variation of the resonance frequency initially disperses or temperature changes, so a narrow band notch filter with a large Q can be used, and little phase delay is generated near the gain cross-over of the servo loop.
However, an actuator generally has a plurality of resonance modes, so it is very difficult to correctly measure a higher resonance frequency to be the problem, from the response to the applied signal, as was proposed above.
When a narrow band notch filter is used, in particular, the attenuation characteristic differs greatly when there is a slight frequency change, so an error in measurement leads to the deterioration of servo characteristics and it is difficult to completely eliminate the influence of higher resonance. Also, to measure a higher resonance frequency, expensive hardware or complicated software are required separately, which increases cost.
In order to implement a servo system which can support the changes of higher resonance frequency by inserting a wide band notch filter with a small Q, where even if the higher resonance frequency of the actuator is close to the gain cross-over frequency and it is unavoidable that the dip frequency of the notch filter and the gain cross-over frequency are close to each other, the following proposal has been made (e.g. Japanese Patent Laid-Open No. H9-44863).
In the control system where a servo error signal is fed back to the actuator via the phase advance compensation circuit and the notch filter so as to create a control loop, the cross-over frequency (polar frequency) at the high frequency side of the phase advance compensation circuit is set to be higher than the frequency whereby the gain of the notch filter becomes the minimum (dip frequency). By this, the phase margin and the gain margin of the control loop can be guaranteed, and a constant and stable servo system can be implemented without complicated hardware and software, even if higher resonance frequency changes occur.
For storage products, such as an optical disk unit, increasing capacity and decreasing price must be pursued. Therefore, current disk units must satisfy two contradictory requirements: one is increasing the positioning accuracy of the head to several tens nm to accurately read and write data, and two, to keep the sampling frequency of the digital filter of the servo control system as low as possible, so that an inexpensive DSP can be used to decrease cost.
Keeping the sampling frequency low, in particular, increases the dead time of a digital filter, and makes the phase conditions of the control system strict, which make band improvement difficult.
According to the conventional tracking control method, as seen in Japanese Patent Laid-Open No. H9-44863 for example, the phases required for the feedback control system are secured by setting the cross-over frequency of the pole of the phase advance compensation to a position which is higher than the dip frequency (a frequency where gain becomes the minimum) of the notch filter. In order to obtain a sufficient phase margin with this method, however, a pole of the phase advance compensation must be set at a area frequency which is higher than the conventional value, so the sampling frequency of the digital filter must be set high. Therefore, it is required a high-speed digital circuit, such as high-speed DSP, and cost is increased.